This invention relates to a method for efficiently removing natural gas liquids from a natural gas stream at an elevated pressure while liquefying the natural gas stream at an elevated pressure.
In recent years the demand for natural gas has increased, particularly in many areas where no natural gas reserves or few natural gas reserves are found. Since many areas have abundant supplies of natural gas it is desirable to be able to transport the natural gas from these areas to market areas. One method for transporting the natural gas is by liquefying the natural gas. Use of liquefied natural gas (LNG) and methods for liquefying natural gas are well known. The natural gas may be liquefied at the point of production or may be liquefied at the point of use when it is available in surplus during portions of the year, i.e., during the summer months when less is required for heating. The natural gas is then readily stored as liquefied natural gas to meet winter peak demand for natural gas in excess of that available through an existing pipeline or the like.
Natural gas is widely used as a fuel and is widely transported as a liquefied natural gas product. The natural gas may be liquefied by a variety of processes, one of which is frequently referred to as a mixed refrigerant process. Such processes are shown, for instance, in U.S. Pat. No. 4,033,735 issued Jul. 5, 1977 to Leonard K. Swenson and in U.S. Pat. No. 5,657,643 issued Aug. 19, 1997 to Brian C. Price. These references are hereby incorporated in their entirety by reference.
In such processes a mixed refrigerant is used in a single heat exchange zone to achieve the desired cooling to liquefy the natural gas.
Other systems, which have been used, are referred to frequently as cascade systems, One such system is shown in U.S. Pat. No. 3,855,810 issued Dec. 24, 1974 to Simon, et al. This reference is also incorporated in its entirety by reference. Such processes utilize a plurality of refrigerant zones in which refrigerants of decreasing boiling points are vaporized to produce a coolant. In such systems, the highest boiling refrigerant, alone or with other refrigerants, is typically compressed, condensed and separated for cooling in a first refrigeration zone. The compressed cool, highest boiling point refrigerant is then flashed to provide a cold refrigerant stream which is used to cool the compressed, highest boiling point refrigerant in the first refrigeration zone. In the first refrigeration zone some of the lower boiling refrigerants may also be cooled and subsequently condensed and passed to vaporization to function as a coolant in a second or subsequent refrigeration zone and the like. As a result, the compression is primarily of the highest boiling refrigerant.
The composition of the natural gas liquids can vary widely from one natural gas source to another. In both types of processes, it is necessary to remove heavier natural gas liquids (C5 +) from the natural gas to prevent plugging of the heat exchange passageways for the natural gas. Also it is often desirable in some instances to recover lighter hydrocarbons, such as C2, C3 and C4. It is often desirable to recover the C2, C3, and C4 hydrocarbons along with the heavier hydrocarbons since they may be more valuable as a separate product or as a part of the natural gas liquids, than as a portion of the LNG. In all instances, however, if substantial quantities of heaver natural gas liquids are present in the natural gas passed to the natural gas liquefaction zone, they freeze in the heat exchange passageways in the refrigerant zone at the liquefaction temperatures and plug the passageways.
In many instances, the natural gas is available at relatively high pressures, i.e., up to and possibly above about 1500 psig. It is much more efficient to liquefy the natural gas at elevated pressure than at lower pressure. Unfortunately the separation of the natural gas liquids and the remaining components of the natural gas stream requires that the pressure of the natural gas stream be reduced to a pressure below about 650 psig to achieve efficient separation of the methane from the remaining components of the natural gas. This results in the return of the natural gas after demethanation to the heat exchange passageways through the refrigeration section at a lower pressure, thereby resulting in liquefaction at the lower pressure. As indicated previously, it is more efficient to liquefy the natural gas at an elevated pressure.
Accordingly, more efficient methods have been sought for removing natural gas liquids from high-pressure natural gas streams without the loss of pressure so that the natural gas can be liquefied at elevated pressure.
According to the present invention, an improved process for efficiently liquefying a natural gas stream having a pressure greater than about 500 psig in a mixed refrigerant process to produce a liquefied natural gas stream is provided. The process comprises cooling the natural gas stream in a heat exchanger in the mixed refrigerant process to a first temperature less than about xe2x88x9240xc2x0 F. to produce a cooled natural gas stream; passing the cooled natural gas stream to a liquid separation zone to produce a first gas stream and a first liquids stream; passing the first liquids stream to a methane separation tower at a temperature less than about xe2x88x9240xc2x0 F. and at a pressure less than about 650 psig to produce a second gas stream containing at least fifty percent methane and a second liquids stream containing natural gas liquids; passing the first gas stream to a turbo expander to reduce the pressure of the first gas stream to a pressure less than about 650 psig to produce a reduced pressure gas stream and passing the reduced pressure gas stream to the methane separation tower; driving a compressor with the turbo expander; passing the second gas stream to the compressor and compressing the second gas stream to a pressure of at least about 500 psig to produce a compressed gas stream; and, passing the compressed gas stream to the heat exchanger for liquefaction at a pressure of at least about 500 psig to The present invention further comprises a process for liquefying a natural gas stream having a pressure greater than about 500 psig in a natural gas liquefaction process to produce a liquefied natural gas stream. The process comprises cooling the natural gas stream in a heat exchanger to a first temperature less than about xe2x88x9240xc2x0 F. to produce a cooled natural gas stream; passing the cooled natural gas stream to a liquid separation zone to produce a first gas stream and a first liquids stream; passing the first liquids stream to a methane separation tower at a temperature less than about xe2x88x9240xc2x0 F. and at a pressure less than about 650 psig to produce a second gas stream containing at least fifty percent methane and a second liquids stream containing natural gas liquids; passing the first gas stream to a turbo expander to reduce the pressure of the first gas stream to a pressure less than about 650 psig to produce a reduced pressure gas stream and passing the reduced pressure gas stream to the methane separation tower; driving a compressor with the turbo expander; passing the second gas stream to the compressor and compressing the second gas stream to a pressure of at least about 500 psig to produce a compressed gas stream; and, passing the compressed gas stream to the heat exchanger for liquefaction at a pressure of at least about 500 psig to produce the liquefied natural gas.
The invention further comprises a system for liquefying a natural gas stream having a pressure greater than about 500 psig, the system comprising: a refrigeration unit adapted to cool the natural gas to a temperature sufficient to liquefy at least a major portion of the natural gas, the refrigeration unit having an intermediate gas outlet, an intermediate gas inlet and a product liquefied natural gas outlet; a separator in fluid communication with the intermediate gas outlet and having a gas outlet and a liquids outlet; a methane separator in fluid communication with the liquids outlet and having an overhead gas outlet, a bottom liquid outlet and a gas inlet; a turbo expander in fluid communication with the gas outlet from the separator and the gas inlet to the methane separator; and, a compressor driven by the turbo expander and in fluid communication with the overhead gas outlet and having a compressed gas outlet in fluid communication with the intermediate gas inlet.
The invention further comprises a process for efficiently separating natural gas liquids from a natural gas stream at a pressure greater than about 500 psig to produce a high pressure gas stream and a natural gas liquid stream by cooling the natural gas stream to a first temperature less than about xe2x88x9240xc2x0 F. to produce a cooled natural gas stream; passing the cooled natural gas stream to a liquid separation zone to produce a first gas stream and a first liquids stream; passing the first liquids stream to a methane separation tower at a pressure less than about 650 psig to produce a second gas stream containing at least fifty percent methane and a second liquids stream containing natural gas liquids; passing the first gas stream to a turbo expander to reduce the pressure of the first gas stream to a pressure less than about 650 psig to produce a reduced pressure gas stream and passing the reduced pressure gas stream to the methane separation tower; driving a compressor with the turbo expander; and, passing the second gas stream to the compressor and compressing the second gas stream to produce a high pressure compressed gas stream.